Unexpected Digital Noise; What's Going On?

Interesting stairwell (click to enlarge)
This stairwell was located in a group of buildings in downtown Tucson, Arizona  The entire building complex
was painted in unusual colors.
Nikon D70, 18-70mm f/3.5-5.6 lens @ 40mm; 1/10 sec. @ f/4.5; ISO Unknown

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On occasion, I’ve noticed digital noise in image files where I don't believe there should be  digital noise.  Files that were made using base ISO (200, in my case) or a relatively low ISO.  The noise I'm seeing is not very obtrusive, but nonetheless present where I don't think any should be seen.  In other words, the noise I see is not like the noise you would expect if shooting at ISO 6400 or such, but is in image files where it really shouldn't appear. I was somewhat mystified as to why.  

I know you are now asking yourself, If it isn't obtrusive or visually disturbing, why am I worried about it?  Well, I'm not worried nor disturbed by the noise, only curious as to its periodic and unexpected cause.  I want to understand why.  Curiosity and understanding drive much of my activities; part of continuous learning.

Thinking about this issue and going back to examine the circumstances under which I made the images in question, I think I may have determined its origin.  The files all come from high speed sequences.  More specifically toward the end of high speed sequences.  The slight noise seems to become visible in files after making a lot of images using a high rate of frames per second (FPS) in a short period of time. Situations such as photographing birds in flight or sports would be good examples of shooting many images in sequence very quickly.

I don't have any science or test equipment to back up my hypothesis, but I’m thinking the noise becomes present after a long, rapid-fire series of exposures because the sensor is starting to heat up.  I’m only speculating here, but it kind of makes sense since photographing at 5, 10 or more FPS is somewhat similar to movie making where we know heat becomes an issue.  It makes sense that heat is generated with continuous use of the sensor.  Again, this is only my guess based upon the limited evidence I have.

Anyone have any alternate ideas for this occasional digital noise in some images but not in others made with similar (base) ISO and camera settings?  I would be interested in hearing from you.

This is just an FYI post.  If you see some unexpected digital noise in your files, this may be a cause.  Then again, it may not be!  Lol.

Thanks for looking. Enjoy! 

Dennis A. Mook 

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